Accelerated breakdown of reticulocyte protein formed under conditions of amino acid depletion

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Abstract
Labile protein was formed when rat or rabbit reticulocytes were incubated in medium deficient in individual amino acids, especially histidine, valine or alanine. The fraction of unstable protein was increased to about 35% of the total protein synthesized when the histidinyl-tRNA-charging inhibitor, histidinol, was added to histidine-deficient media. The MW of the labile proteins measured by sodium dodecyl sulfate/polyacrylamide-gel electrophoresis with urea were less than Hb and probably represent prematurely terminated Hb chain. Although protein synthesis was always lower under conditions that produce labile protein, inhibition of protein synthesis by fluoride or cycloheximide did not give an effect similar to amino acid depletion. The synthesis of protein in deficient medium did not alter the degradation rate of pre-existing protein in reticulocytes and was thus unrelated to the stringent response in bacteria. Amino acid-deficient medium apparently leads to a decreased charging of the appropriate tRNA, a concomitant decrease in protein synthesis and the degradation of nascent peptides.